The invention relates to a direct-conversion receiver for radio communication systems such as portable cellular phones, cordless phones, pagers, etc.
The first generation of cellular systems relied on analog frequency modulation for speech transmission, and several standards have been developed, e.g., NMT 450, NMT 900, AMPS, and ETACS.
The second generation of cellular systems, e.g., the Global System for Mobile communications (GSM) in Europe and the American Digital Cellular System (ADC) in North America, employ digital voice transmission and some digital services, such as facsimile and short message services.
Receivers in cellular systems and the other fields noted above are preferably small, lightweight, and inexpensive. To make a portable receiver like a hand-held telephone smaller and less expensive, much research has been done to increase the level of integration of different parts of the phone. But previous receivers have been of the conventional heterodyne type. For applications in small, low-cost mobile communication systems, such receivers suffer from high production costs caused by expensive and non-integrable components, such as bandpass filters.
To overcome such drawbacks, an alternative receiver architecture has been developed that is based on the direct-conversion principle, in which the frequency of the local oscillator is the same as the frequency of the received radio carrier. Consequently, the received radio signal is down-converted directly to base band in one step. Since a direct-conversion receiver does not have any intermediate frequency (IF) stages, many filters can be omitted or simplified.
Direct conversion was introduced for single-sideband receivers in the 1950's, but the technique is not limited to such systems. Direct conversion can be used with many different modulation schemes and is especially well suited for the quadrature modulation schemes of today, such as minimum shift keying (MSK) and quadrature amplitude modulation (QAM). Various aspects of direct-conversion or homodyne receivers are described in U.S. patent application Ser. No. 08/303,183 entitled "Radio Receiver" by two of the current Applicants.
The operation of a conventional direct-conversion receiver can be described as follows with reference to FIG. 1a. A radio frequency (RF) signal having center frequency f.sub.c and bandwidth BW.sub.rf is received by an antenna 10 and then is filtered by a bandpass filter 20. The filtered signal produced by the bandpass filter is amplified by an amplifier 30, which preferably has low noise to improve the total noise figure of the receiver.
The amplified filtered signal produced by the amplifier 30 is then down-converted to base band in an in-phase (I) channel and a quadrature phase (Q) channel by balanced mixers 40, 50. The mixers are driven by respective ones of sine (I) and cosine (Q) components produced from a sinusoidal signal generated by a local oscillator 60 by a suitable divider and phase shifter 70. According to the direct-conversion principle, the LO signal also has the frequency f.sub.c.
The mixers 40, 50 effectively multiply the signal from the amplifier 30 and the I and Q components of the local oscillator. Each mixer produces a signal that has frequencies that are the sum and difference of the frequencies of the amplified filtered received signal and the local oscillator signal. The difference (down-converted) signals each have a spectrum that is folded over around zero frequency (d.c.) and that spans from d.c. to 1/2 BW.sub.rf.
The I and Q signals produced by the mixers are filtered by low-pass filters 80, 90 that remove the sum (up-converted) signals, as well as components that might be due to nearby RF signals. The filters 80, 90 set the noise bandwidth and thus the total noise power in the receiver. The I and Q base band signals are then usually amplified by amplifiers 100, 110, and provided to further processing components that produce the demodulated output signal. Such further processing can include phase demodulation, amplitude demodulation, frequency demodulation, or hybrid demodulation schemes.
A major problem with the direct-conversion receiver is that second-order products of interferers (e.g., signals on the same and nearby RF communication channels) are produced by the mixers. One component of these second-order products is located at base band, and thus interferes with the desired base band signal, degrading performance. In some situations, this problem totally blocks communication in high-performance, direct-conversion receivers for today's time division multiple access (TDMA) digital cellular systems.
For an input signal V.sub.in, a non-linear device, such as a mixer, will produce an output signal V.sub.out theoretically given by the following expression: EQU V.sub.out =aV.sub.in +bV.sub.in.sup.2 + (1)
If the input signal V.sub.in is an interfering signal given by: EQU V.sub.in =V.sub.m cos(.omega..sub.c t) (2)
where V.sub.m is the interferer's maximal amplitude and .omega..sub.c corresponds to the carrier frequency f.sub.c, the second-order product bV.sub.in.sup.2 is given by: ##EQU1## It is clear from Eq. 3 that the first term on the right is a distortion on the desired signal at base band, e.g., after the mixers 40, 50. The second term on the right can be neglected since it represents the up-converted (sum) signal centered around twice the carrier frequency that is removed by the filters 80, 90.
The distortion is a d.c. component if the interfering signal is either only a single carrier f.sub.c or a constant-envelope, frequency- or phase-modulated signal. Such a d.c. offset can be removed, for example, in the manner described in U.S. Pat. No. 5,241,702 to Dent, which is hereby expressly incorporated by reference in this application.
If the interferer is in some way an amplitude-modulated (AM) signal, viz., if V.sub.m is not a constant, the second-order product no longer simply introduces a d.c. offset but distortion in the frequency band (d.c. to 1/2 BW.sub.rf) of interest. This happens in all digital communication systems due to their use of real AM signals and/or to their use of on/off switching of single-carrier or frequency- or phase-modulated signals. Although direct-conversion receivers are known, none shows how to cope with the high second-order products of the above-described interferers.
Today, direct conversion is not used for high performance cellular mobile receivers. If it were used, however, a large ratio between the desired signal and the interferers and/or a high second-order intercept point (&gt;60 dBm) would be required. It is currently believed the direct-conversion solution is not practical for systems such as ADC, GSM, and DSC 1800 in which these high requirements apply, but direct conversion could be used in systems such as pagers and DECT in which the second-order intercept point requirement is much lower.